camera dpi question

[EOS_JD]

thanks for the advice and i understand the pixels per inch thing but i don't understand why in adobe CS2, bridge, photos shot with the 20D show up as 240dpi and the 30D photos show up as 300 dpi. i don't get it. and as far as printing and dividing by some number 3000 X whatever, i'm just lost. i have a mental block with it. my biggest thing was...why do the 20D and 30D show up as different dpi, ppi, whatever, in adobe bridge?
steve

OK back to basics. You have an image that is 300pixels x 300 pixels. In prder to print this image at 1" in size I need to print at 300pixels per inch. This means 300 pixels horizontally and vertically. At any other ppi setting it will not print at 1".

Say you take this 300pixel x 300pixel image reduce the ppi setting to 100ppi.

What size will it print now? 300/100 = 3" square (both axis are same size).

If you print the 300pixel x 300pixel image at 600ppi it would print at 1/2"

3 equations

Print Size = PS
No or pixels = P
Resolution = PPI

So

To find out PS = PPI/P
To find out R = P/PS
To find out P = PS x PPI

Use these 3 equations

I appreciate your pointg regards the camera setting and to be honest it's probably down to Canon or the way photoshop reads the 20D/30D files but with the above info, the resolution really means nothing (honestly). It only matters when you come to print.

If you shoot RAW you can set up Adobe Camera Raw to output at any resolution and it'll do this as a default.

Play around with your images in image resize. I shjould ask that it's best to keep the "resample" button unchecked. If you leave this on, you will add or subtract pixels which changes the physical image you captured. Unchecking this will not change the actual image, only the actual print size.

 

[EOS_JD]

To prove it to yourself, import an image from each camera and change that number to 72 - each image will look identical on the screen.

Yes but make sure resample is unchecked.

Standard prints are printed at 300 dots per inch (different term from pixels per inch). Dots per inch makes a big difference to the final print - pixels per inch doesn't.
You want a 12 x 8 inch print. So if standard printing is 300 dpi (note dpi when talking about printing) then you'll need 12 (x300) x 8 (x300) to achieve a nice print. In other words your original image would have to be 3600 x 2400.

David I'm afraid you too are getting these mixed up.

Most labs do indeed print at 300dpi. On my epson R2400 I can print at over 5000dpi. You too are confusing dpi and ppi. It is 12x300ppi. Only actual physical printer produces the dots. You cannot set this in photoshop. You set printer resolution in the print menu (when printing) or the lab does this themselves.

Dots per inch makes a big difference to the final print - pixels per inch doesn't.

The quality of an image depends on both. Try printing a 6x4 at 72ppi. I can still print this 72ppi image at 300dpi but it'll still look crap.

Quality is in the pixels per inch (240ppi up to 300ppi should be fine for any image up to A4 in size. As you get bigger prints you can in fact print at lower ppi settings because you are viewing the image from slightly further away so you will not see any drop in quality.

I print my A3+ files at 180ppi and at 5760ppi on my R2400

The more pixels you pack into an inch of print size, the higher the quality of the output will be. 300ppi is generally as high as you would ever need as the eye can't distinguish any higher although I regularly print my images at less as nboted above - just depends on the size of my final output image.. Try setting your image to 240ppi and printing. You'll not see any difference.

You'd need to adjust your jpeg image to 3600 x 2400 using photoshop. So you'd be changing the resolution or "image size" as PS calls it.

Changing resolution does not change the image size. It only chjanges the print size. If you have a 3504px x 2336px image and chane the res from 300ppi to 72ppi, the image size is still 3504px x 2336px all that's changed is the print size. the image will remain unchanged so long as you leave resample unchecked.

The important thing is that the image you send to the lab is suitable for printing at the size you specify and simply multiplying both dimensions by 300 and adjusting to that size is the quickest and easiest way to do it.

Hope that clears things up a bit for you

Again you do NOT require to set 300ppi as the default. Sure if you have a small enough image (on a 20D that equates to roughly 12" x 8") then you can print at 300ppi no probblem but as you gop larger dropping the ppi setting to default) will not show much (if any) loss of image detail.

David
It's amazing the number of people that think they have it and although you are probably doing this right, you've explained it wrongly. I hope this helps you all understand?

I might not be explaining too well and if there's anything you don't understand please give me a shouyt. I'm passionate about this point because once you fully understand the concept your workflow will be so much easier.

Regards
Jim

 

[guitarkid]

thanks for clearing that up and thanks for your patience! so if i want an 8X10 print at 300 dpi i would go 8 X 300 and 10 X 300 so i would need to set res to 2400 X 3000. looks like i would multiply by 300. but then again, as you said, PS does all this for me and i never resample or resize. the only time i do that is if i'm sending some photos to family or friends and then i resize to 640 X 480 for easy emailing.

steve

 

[darich]

Jim - you're right...I thought I had but i can't quite follow a couple of things you said. I'll do it publicly on the forum so others can hopefully benefit too.
First off -

Yes but make sure resample is unchecked - agreed - I should have mentioned that.

Most labs do indeed print at 300dpi. On my epson R2400 I can print at over 5000dpi. You too are confusing dpi and ppi. It is 12x300ppi. Only actual physical printer produces the dots. You cannot set this in photoshop. You set printer resolution in the print menu (when printing) or the lab does this themselves.
Changing resolution does not change the image size. It only chjanges the print size


Let's say i take an image straight from my camera - 4368 x 2912. If i want a 30 inch by 20inch print then you're saying it'll look identical whether i change the image size in photoshop or not and it also doesn't matter whether i work at 72ppi or 300ppi???

When i want an image that size i upsize it to 9000 x 6000 pixels so that when printed i know it'll look ok.
Does that mean i don't need to upsize?

Maybe I do have it but i didn't explain it very well and used a couple of incorrect terms!!

 

[EOS_JD]

thanks for clearing that up and thanks for your patience! so if i want an 8X10 print at 300 dpi i would go 8 X 300 and 10 X 300 so i would need to set res to 2400 X 3000. looks like i would multiply by 300. but then again, as you said, PS does all this for me and i never resample or resize. the only time i do that is if i'm sending some photos to family or friends and then i resize to 640 X 480 for easy emailing.

steve

Yes and no. By resampling you are reducing the size of your image. Setting the pixel size IS resampling. What I'd do is uncheck resample and set the size of the image.

I have a 20D with 3504 x 2336 pixels. If I want a 10x8 I select the crop tool; and set the size in the bar along the top and crop to the size I want. I leave resolution blank and it gives me the res automatiucally.

I should add using the crop tool to resize images is the easiest and quickest way.

Setting the 640x 480 is correct depending on what ratio the image is. 600x400 is my usual for a 6x4 ratio image.

 

[EOS_JD]

Jim - you're right...I thought I had but i can't quite follow a couple of things you said. I'll do it publicly on the forum so others can hopefully benefit too.
First off -

Yes but make sure resample is unchecked - agreed - I should have mentioned that.

Most labs do indeed print at 300dpi. On my epson R2400 I can print at over 5000dpi. You too are confusing dpi and ppi. It is 12x300ppi. Only actual physical printer produces the dots. You cannot set this in photoshop. You set printer resolution in the print menu (when printing) or the lab does this themselves.
Changing resolution does not change the image size. It only chjanges the print size


Let's say i take an image straight from my camera - 4368 x 2912. If i want a 30 inch by 20inch print then you're saying it'll look identical whether i change the image size in photoshop or not and it also doesn't matter whether i work at 72ppi or 300ppi???

When i want an image that size i upsize it to 9000 x 6000 pixels so that when printed i know it'll look ok.
Does that mean i don't need to upsize?

Maybe I do have it but i didn't explain it very well and used a couple of incorrect terms!!

First off the image itself on the screen will be identical no matter the resolution

However I'd not advise printing at 72ppi as (depending on the image size) it'll probably look crap (see my post above).

Let's say i take an image straight from my camera - 4368 x 2912. If i want a 30 inch by 20inch print then you're saying it'll look identical whether i change the image size in photoshop or not and it also doesn't matter whether i work at 72ppi or 300ppi???

First by resampling you are adding pixels that are not required (some may disagree with this - try comparing two).

OK with 4368 x 2912 pixels and you want to print at 30x20, this would give a default res of 145.6ppi. This should be fine for a print at this size - however I normally resize to 180ppi as a minimum just adding a small number of pixels. You are doubling the number of pixels in the image and the computer in randomly generating those pixels.

YTry printing one at the default res and see if you can see a difference? If you have a printer, make a 100% crop of part of each image and use your home printer to check it out.

There woun't be a massive difference (if any) in the output but the file sizes you are working with are massive and huge drain on PC resources.

The reason that you'll not see any difference is that you look at small images like 6x4s from a few inches. You need the highest res to get the best quality but printing a 30x20, you'll look at this from a few meters away and it'll not look any different really - just a great looking image so printing at even 150ppi should look fine.